1. Field of the Invention
This invention relates in general to subsea oilwell installations, and in particular to a means for purging a completion workover riser string of oil preparatory to pulling the riser string.
2. Description of the Prior Art
In general, a subsea well assembly has a Christmas tree mounted on a wellhead. The wellhead comprises the upper end of casing which has been cemented in the well. A tubing hanger is mounted in the wellhead, with the oil being produced through the tubing normally. At times the operator may need to pump stimulating or cleaning fluids into the well or perform wireline operations to increase production. During completion and workover operations, the operator removes the cap to the Christmas tree and lowers a workover riser string from a workover vessel to the Christmas tree. The riser string has a central passage for pumping fluids down and also has an annulus passage which is in communication with an annulus passage in the Christmas tree. The annulus passage in the Christmas tree communicates with the annulus between the tubing and the casing in the well.
When completion or workover operations are taking place, the riser string production passage may be filled with reservoir fluid or treating chemicals. To avoid polluting the sea when pulling the riser string, a purge valve is mounted externally above the Christmas tree. This purge valve will enable fresh water to be circulated up the production passage of the string, purging the string of pollutants. While this system achieves the desired results, external purge valves are difficult to mount, adversely affect the balance of the completion workover package and expensive.